Please can someone explain to me what Spelling age and reading age is ?

[PostmanPatsBlackandWhiteCat]

Please can someone explain to me what Spelling age and reading age is ?
The relationship between the two and how they are calculated.
My DD Y4 has a reading of 10.3 years and a spelling age of 13.I was wondering why the gap is so big

[learnandsay]

You'd probably need to know which tests she took to know how accurate a reflection of her abilities the results are, but regardless it looks as though she spells very well.